Always include a plain-text version of your message. It's perfectly acceptable to use a template or type in your own HTML message, but if you do so, also include a plain-text version.

Your message should not consist of one large image. Spammers commonly send just an image with a small amount of text added to try to bypass filters. Your message should be composed of mostly text if you include images.

Encourage your subscribers to add you to their address book. Some ISPs and email applications will place a message in an inbox always when this is the case.

Test your messages. Sign up for some free accounts and send yourself tests as an actual subscriber to your own list. Review how they're handled by those ISPs in particular to establish how it might be handled among others.